Chapter Description

Variable-length subnet masks (VLSM) simply means that the subnet design uses more than one mask in the same classful network. VLSM has some advantages and disadvantages, but when learning, the main challenge is that a subnetting design that uses VLSM requires more math, and it requires that you think about some other issues as well. This chapter walks you through the concepts, the issues, and the math.

IPv4 addressing and subnetting use a lot of terms, a lot of small math steps, and a lot of concepts that fit together. While learning those concepts, it helps to keep things as simple as possible. One way this book has kept the discussion simpler so far was to show examples that use one mask only inside a single Class A, B, or C network.

This chapter removes that restriction by introducing variable-length subnet masks (VLSM). VLSM simply means that the subnet design uses more than one mask in the same classful network. VLSM has some advantages and disadvantages, but when learning, the main challenge is that a subnetting design that uses VLSM requires more math, and it requires that you think about some other issues as well. This chapter walks you through the concepts, the issues, and the math.

“Do I Know This Already?” Quiz

Take the quiz (either here, or use the PCPT software) if you want to use the score to help you decide how much time to spend on this chapter. The answers are at the bottom of the page following the quiz, and the explanations are in DVD Appendix C and in the PCPT software.

Which of the following routing protocols support VLSM? (Choose three answers.)

RIPv1

RIPv2

EIGRP

OSPF

What does the acronym VLSM stand for?

Variable-length subnet mask

Very long subnet mask

Vociferous longitudinal subnet mask

Vector-length subnet mask

Vector loop subnet mask

R1 has configured interface Fa0/0 with the ip address 10.5.48.1 255.255.240.0 command. Which of the following subnets, when configured on another interface on R1, would not be considered an overlapping VLSM subnet?

10.5.0.0 255.255.240.0

10.4.0.0 255.254.0.0

10.5.32.0 255.255.224.0

10.5.0.0 255.255.128.0

R4 has a connected route for 172.16.8.0/22. Which of the following answers lists a subnet that overlaps with this subnet?

172.16.0.0/21

172.16.6.0/23

172.16.16.0/20

172.16.11.0/25

A design already includes subnets 192.168.1.0/26, 192.168.1.128/30, and 192.168.1.160/29. Which of the following subnets is the numerically lowest subnet ID that could be added to the design, if you wanted to add a subnet that uses a /28 mask?